AK-pattern firearms, including the AK-47, AKM, AK-74, and AK-100, are widely appreciated for their reliability, relatively low production costs, and ease of use. The construction and operation of AK-pattern firearms are well-known, but a brief description of the gas piston operating system will provide context for a later description of the present invention. Referring to FIGS. 1 and 2, therein are shown the relevant elements of a prior art AK-pattern gas piston system. A gas block 10 is mounted on a barrel 12. The bore 14 includes a gas port 16 aligned with a gas passageway 18 in the gas block 10. A gas piston 20 includes a head portion 22 that is received within a cylindrical chamber 24 of the gas block 10. At the opposite end of the gas piston is a connector portion 26 which is attached to a forward extension of the bolt carrier assembly (not shown) in a well-known manner. A gas tube 28 acts as a guide for the piston 20, rather than as a pressure chamber. In some models, the gas tube 28 has embossed flutes (not shown) that more closely guide the piston 20. This system is known as a “long stroke” gas piston, because the piston travels the same, full distance as the bolt carrier each time it cycles.
As a projectile is propelled through the bore 14 of the barrel 12, propellant gas flows through the gas port 16 and gas passageway 18 of the gas block 10 into the cylindrical chamber 24 to exert a force in the form of expanding gas pressure against the head 22 of the piston 20. The gas piston 20 is moved rearwardly by this force, which is transferred to the bolt carrier to cycle the action of the firearm in a well-known manner. The flow of gas through the gas port 16 and gas passageway 18 is unregulated, other than by the selected diameter of these passageways. AK-pattern firearms are sometimes considered to be “over-gassed,” meaning that the flow and pressure of expanding gases exerted against the gas piston 20 are more than adequate to cycle the action. This over-gassing ensures that the firearm cycles reliably, but it may cause punishing wear to moving parts of the firearm. Adding a noise suppressor to the muzzle of the barrel 12 will increase back pressure in the bore 14 and other parts of the gas operating system, resulting in the action cycling too fast or with excessive force.
The nature and simplicity of the AK-pattern gas operating system makes the flow and pressure of operating gases difficult to regulate, at least without complete replacement of the gas block. Others have proposed using a partially vented piston head to allow bypass flow of gases to reduce the force exerted on the gas piston. However, there remains a need for a device that may be added to an existing AK-pattern firearm to adjustably regulate flow and pressure of operating gas without replacement of the gas block.